Heresies: Against Progress and Other Illusionsby John Gray200pp, Granta, £8.99

Gary Larson, one of the geniuses of our time, has a cartoon of a field of sheep all grazing peacefully, all, that is, save one wise ovine, who has lifted its head in appalled astonishment to cry out: "Wait! This is grass - this is grass we're eating!" John Gray has always been the odd-sheep-out. He is probably best-known for his book Straw Dogs: Thoughts on Humans and Other Animals, the title of which is taken from a saying by Lao Tzu: "Heaven and earth are ruthless, and treat the myriad creatures as straw dogs." False Dawn, a debunking of the theory of globalisation, was a global success, having first been dismissed as mere pessimism and later accepted as a soberly realistic assessment of the state of international economics. When the bubble of the 1990s burst with the turn of the millennium, the herd had to admit that yes, it really is grass.

Gray has written a fine study of his friend, the late Isaiah Berlin, whose pluralist philosophy he may be said to develop and refine in his own writing, and, one presumes, in his seminars at the LSE, where he is professor of European thought. He makes large claims for Berlin's work, animated as it is, he writes, "by a single idea of enormous subversive force". This is the idea, which I call value-pluralism, that ultimate human values are objective but irreducibly diverse, that they are conflicting and often uncombinable, and that sometimes when they come into conflict with one another they are incommensurable; that is, they are not comparable by any rational measure.

Certainly this is Gray's own attitude to the warring ideologies of our time. He is richly dismissive, for instance, of the Bush administration's neo-conservatives - "Washington's new Jacobins", he calls them - who believe that it is possible to eradicate evil from the world. "The danger of American foreign policy," he writes, "is not that it is obsessed with evil but that it is based on the belief that evil can be abolished." Such foolishness, he points out, is far removed from the wisdom of America's founding fathers, for whom "the purpose of government was not to conduct us to the Promised Land but to stave off the recurrent evils to which human life is naturally prone".

Gray is an unwavering post-humanist. When we were little, the Catholic catechism used to assure us that God made the world for man's use and benefit, and that therefore we are the lords of creation, with all nature, its flora and fauna, entirely at our command. For liberal humanists, this good news is still good news, but comes in a different bulletin. According to Gray, the so-called secular systems by which we in the west are ruled are in fact the products of spilt religion, as was the Enlightenment faith in the possibility of progress, "the belief that human life becomes better with the growth of knowledge".

Gray evinces a Swiftian contempt for our latter-day lay priestlings, direct heirs of the 18th-century philosophes who proclaimed a new paganism but were in fact neo-Christians, "missionaries of a new gospel more fantastical than anything in the creed they imagined they had abandoned". All the Enlightenment did was to promote religion by other means, and its belief in progress was only the Christian message "emptied of transcendence and mystery". One of the heresies promulgated by Gray is that many of those who today continue to hold to religious faith are far more profound in their thinking, and certainly better educated, than most of their liberal-humanist opponents.

Gray sees our faith in progress - "the Prozac of the thinking classes" - as the illusion that underlies the most egregiously mistaken political and social policies of the present day. Certainly there is such a thing as progress, but it is a fact only in the realm of science, while "in ethics and politics it is a superstition". Throughout his work Gray hammers relentlessly against the notion, first advanced in the Renaissance and reified in the Enlightenment, that history moves inexorably in a straight line, and that human nature will necessarily improve as our knowledge accumulates. He grants that in some areas things do get better: we have abolished judicial torture, for example, and modern dentistry is a great boon. The mistake, he contends, the wilful, foolish and tragic mistake, is to imagine that more dental implants and fewer thumbscrews will make us into better beings. "Human knowledge grows, but the human animal stays much the same."

Clear thinking is always a bracer, but does Gray as Cassandra have anything to offer other than an injunction to look all gift horses in the mouth? He is a stoic and, on occasion, even a meliorist, though a highly cautious one. All we can do, he declares, is to try to curb the wilder hungers of Homo rapiens and work away piecemeal at containing the forces that seek to destroy us. The so-called war on terror, in which "Dr Strangelove has joined forces with Dr Billy Graham", he takes as one of our more dangerously deluded enterprises. Granted, al-Qaida is a threat to the very continuance of our liberal values in the west, but it is the height of foolishness to pretend that we can defeat it by force of arms. Instead of invading Iraq and making threatening noises against Syria and Iran, the sensible policy would be to address regional conflicts, such as the Israeli-Palestinian blood feud, and separate them from the activities of al-Qaida with the aim of returning terrorism to "more historically normal levels". The new crusaders in the White House and the Pentagon, of course, would be contemptuous of such a gradualist approach.

Heresies collects essays that Gray wrote for the New Statesman from the late 1990s through November of last year. The pieces have been lightly edited, he tells us, and no substantial changes have been made. The prescience of his views on such topics as Iraq and Tony Blair's political career is remarkable. One does wonder what the magazine's readers made of the contention that Donald Rumsfeld's Hobbesian pragmatism is to be preferred to Bill Clinton's impulsiveness, that "in intellectual terms atheism is a Victorian fossil", or the baleful but gracefully expressed reminder that "the human animal is itself only a passing tremor in the life of the planet". All flesh is as grass, indeed.

· John Banville's Prague Pictures: Portrait of a City is published by Bloomsbury.